Advanced automotive battery development for vehicle propulsion has, in the past, been directed primarily at the requirement of fully electric propulsion systems for such vehicles. To this end, Stanford Ovshinsky and his battery development teams at Energy Conversion Devices, Inc. and Ovonic Battery Company have made great advances in nickel-metal hydride battery technology for such applications.
Initially effort focused on metal hydride alloys for forming the negative electrodes of such batteries. As a result of their efforts, they were able to greatly increase the reversible hydrogen storage characteristics required for efficient and economical battery applications, and produce batteries capable of high density energy storage, efficient reversibility, high electrical efficiency, efficient bulk hydrogen storage without structural changes or poisoning, long cycle life, and repeated deep discharge. The improved characteristics of these highly disordered "Ovonic" alloys, as they are now called, results from tailoring the local chemical order and hence the local structural order by the incorporation of selected modifier elements into a host matrix.
Disordered metal hydride alloys have a substantially increased density of catalytically active sites and storage sites compared to single or multi-phase compositionally homogeneous crystalline materials. These additional sites are responsible for improved efficiency of electrochemical charging/discharging and an increase in electrical energy storage capacity. The nature and number of storage sites can even be designed independently of the catalytically active sites. More specifically, these alloys are tailored to allow bulk storage of the dissociated hydrogen atoms at bonding strengths within the range of reversibility suitable for use in secondary battery applications.
Some extremely efficient electrochemical hydrogen storage materials were formulated, based on the disordered materials described above. These are the Ti-V-Zr-Ni type active materials such as disclosed in U.S. Pat. No. 4,551,400 ("the '400 Patent") to Sapru, Hong, Fetcenko, and Venkatesan, the disclosure of which is incorporated by reference. These materials reversibly form hydrides in order to store hydrogen. All the materials used in the '400 Patent utilize a generic Ti-V-Ni composition, where at least Ti, V, and Ni are present and may be modified with Cr, Zr, and Al. The materials of the '400 Patent are multiphase materials, which may contain, but are not limited to, one or more phases with C.sub.14 and C.sub.15 type crystal structures.
Other Ti-V-Zr-Ni alloys are also used for rechargeable hydrogen storage negative electrodes. One such family of materials are those described in U.S. Pat. No. 4,728,586 ("the '586 Patent") to Venkatesan, Reichman, and Fetcenko, the disclosure of which is incorporated by reference. The '586 Patent describes a specific sub-class of these Ti-V-Ni-Zr alloys comprising Ti, V, Zr, Ni, and a fifth component, Cr. The '586 Patent, mentions the possibility of additives and modifiers beyond the Ti, V, Zr, Ni, and Cr components of the alloys, and generally discusses specific additives and modifiers, the amounts and interactions of these modifiers, and the particular benefits that could be expected from them.
In contrast to the Ovonic alloys described above, the older alloys were generally considered "ordered" materials that had different chemistry, microstructure, and electrochemical characteristics. The performance of the early ordered materials was poor, but in the early 1980's, as the degree of modification increased (that is as the number and amount of elemental modifiers increased), their performance began to improve significantly. This is due as much to the disorder contributed by the modifiers as it is to their electrical and chemical properties. This evolution of alloys from a specific class of "ordered" materials to the current multicomponent, multiphase "disordered" alloys is shown in the following patents: (i) U.S. Pat. No. 3,874,928; (ii) U.S. Pat. No. 4,214,043; (iii) U.S. Pat. No. 4,107,395; (iv) U.S. Pat. No. 4,107,405; (v) U.S. Pat. No. 4,112,199; (vi) U.S. Pat. No. 4,125,688 (vii) U.S. Pat. No. 4,214,043; (viii) U.S. Pat. No. 4,216,274; (ix) U.S. Pat. No. 4,487,817; (x) U.S. Pat. No. 4,605,603; (xii) U.S. Pat. No. 4,696,873; and (xiii) U.S. Pat. No. 4,699,856. (These references are discussed extensively in U.S. Pat. No. 5,096,667 and this discussion is specifically incorporated by reference). Additional discussion is contained in the following U.S. patents, the contents of which are specifically incorporated by reference: 5,104,617; 5,238,756; 5,277,999; 5,407,761; 5,536,591; 5,506,069; and 5,554,456.
Ovshinsky and his teams next turned their attention to the positive electrode of the batteries. The positive electrodes today are typically pasted nickel electrodes, which consist of nickel hydroxide particles in contact with a conductive network or substrate, preferably having a high surface area. There have been several variants of these electrodes including the so-called plastic-bonded nickel electrodes which utilize graphite as a microconductor and also including the so-called foam-metal electrodes which utilize high porosity nickel foam as a substrate loaded with spherical nickel hydroxide particles and cobalt conductivity enhancing additives. Pasted electrodes of the foam-metal type have started to penetrate the consumer market due to their low cost and higher energy density relative to sintered nickel electrodes.
Conventionally, the nickel battery electrode reaction has been considered to be a one electron process involving oxidation of divalent nickel hydroxide to trivalent nickel oxyhydroxide on charge and subsequent discharge of trivalent nickel oxyhydroxide to divalent nickel hydroxide. However, quadrivalent nickel is also involved in the nickel hydroxide redox reaction, the utilization of which has never been investigated.
In practice, electrode capacity beyond the one-electron transfer theoretical capacity is not usually observed. One reason for this is incomplete utilization of the active material due to electrical isolation of oxidized material. Because reduced nickel hydroxide material has high electrical resistance, the reduction of nickel hydroxide adjacent the current collector forms a less conductive surface that interferes with the subsequent reduction of oxidized active material that is farther away.
Ovonic Battery Company has developed positive electrode materials that have demonstrated reliable transfer of more than one electron per nickel atom. Such stable, disordered positive electrode materials are described in U.S. Pat. Nos. 5,344,728; 5,348,822; 5,523,182; 5,569,563; and 5,567,599; the contents of which are specifically incorporated by reference.
As a result of this research into the negative and positive electrode active materials, the Ovonic Nickel Metal Hydride (NiMH) battery has reached an advanced stage of development for EVs. Ovonic electric vehicle batteries are capable of propelling an electric vehicle to over 370 miles (due to a specific energy of about 90 Wh/Kg), long cycle life (over 1000 cycles at 80% DOD), abuse tolerance, and rapid recharge capability (up to 60% in 15 minutes). Additionally, the Ovonic battery has demonstrated higher power density when evaluated for use as an EV stored energy source.
As an alternative to true electric vehicles, hybrid-electric vehicles (HEVs) have gained popularity as having the technical capability to meet the goal of tripling auto fuel economy in the next decade. Hybrid electric vehicles utilize the combination of a combustion engine and an electric motor driven from a battery and have been proposed in a variety of configurations.
Hybrid systems have been divided into two broad categories, namely series and parallel systems. In a typical series system, an electric propulsion motor is used to drive the vehicle and the engine is used to recharge the battery. In the parallel system, both the combustion engine and the electric motor are used to drive the vehicle and can operate in parallel for this purpose.
There are further variations within these two broad categories. For example, there are systems which employ a combination of the series and parallel systems. In the so-called "dual mode" system, the propulsion mode can be selected, either by the operator or by a computer system, as either an "all electric" or "all engine" mode of propulsion. In the "range extender" system, a primarily electric system is used for propulsion and the engine is used for peak loads and/or for recharging the battery. In the "power assist" system, peak loads are handled by the battery driven electric motor.
A further division is made between systems which are "charge depleting" in the one case and "charge sustaining" in another case. In the charge depleting system, the battery charge is gradually depleted during use of the system and the battery thus has to be recharged periodically from an external power source, such as by means of connection to public utility power. In the charge sustaining system, the battery is recharged during use in the vehicle, through regenerative braking and also by means of electric power supplied from a generator driven by the engine so that the charge of the battery is maintained during operation.
There are many different types of systems that fall within the categories of "charge depleting" and "charge sustaining" and there are thus a number of variations within the foregoing examples which have been simplified for purposes of a general explanation of the different types. However, it is to be noted in general that systems which are of the "charge depleting" type typically require a battery which has a higher charge capacity (and thus a higher specific energy) than those which are of the "charge sustaining" type if a commercially acceptable driving range (miles between recharge) is to be attained in operation. Further and more specific discussion of the various types of HEV systems, including "series", "parallel" and "dual mode" types, and of the present invention embodied in such systems will be presented below.
In the present application, the phrase "combustion engine" is used to refer to engines running off of any known fuel, be it hydrogen or hydrocarbon based such as gasoline, alcohol, or natural gas, in any combination.
The use of hybrid drive systems offers critical advantages for both fuel economy and ultra-low emissions. Combustion engines achieve maximum efficiency and minimal emissions when operated at or near the design point speed and load conditions. Small electric motors are capable of providing very high peak torque and power. Thus, the ability to use a small combustion engine operating at maximum efficiency coupled with an electric motor operating at maximum efficiency offers an outstanding combination for minimizing emissions, providing excellent fuel economy, and maximizing acceleration.
A key enabling requirement for HEV systems is an energy storage system capable of providing very high peak power combined with high energy density while at the same time accepting high regenerative braking currents at very high efficiency. In addition, the duty cycle of a peak power application requires exceptional cycle life at low depths of discharge, particularly in charge depleting systems.
It is important to understand the different requirements for this energy storage system compared to those for a pure electric vehicle. Range is the critical factor for a practical EV, making energy density the critical evaluation parameter. Power and cycle life are certainly important, but they are secondary to energy density for an EV. A lightweight, compact, high-capacity battery is the target for pure EV applications.
In contrast, in HEV applications, gravimetric and volumetric power density is the overwhelming consideration. Excellent cycle life from 30 to 60% DOD is also more critical than cycle life at 80% DOD as required in EV applications. Similarly, rapid recharge is also essential to allow efficient regenerative braking, and charge/discharge efficiency is critical to maintain battery state of charge in the absence of external charging. In addition, thermal management and excellent gas recombination are important secondary considerations to rapid recharging and multiple cycling.
Heat generated during charging and discharging NiMH batteries is normally not a problem in small consumer batteries or even in larger batteries when they are used singly for a limited period of time. On the other hand, batteries used in HEVs will be subjected to many rapid charge and discharge cycles during normal operation. Such rapid charging and discharging will result in significant thermal swings that can affect the battery performance. The prior art suggests a variety of solutions to this problem, such as the following:
U.S. Pat. No. 4,115,630 to Van Ommering, et al., describes a metal oxide-hydrogen battery having bipolar electrodes arranged in a centrally drilled stack. This patent describes conducting heat generated in the electrode stack via the hydrogen gas of the cell. In particular, the application notes that because heat conduction perpendicular to electrode plates is 10-20 times smaller than conduction parallel to electrode plates, cells using flat electrodes must be modified significantly which makes them unacceptably heavy.
J. Lee, et al. describe resistive heating and entropy heating in lead-acid and nickel/iron battery modules in 133(7) JESOAN 1286 (July, 1986). This article states that the temperature of these batteries is due to resistive heating and entropy changes of the electrochemical reactions often varies considerably during their operation. They note that the thermal resistance caused by the cell case plays an important role as the cell temperature becomes higher. This reference suggests that some additional cooling structure must be added to the battery.
U.S. Pat. No. 4,865,928 to Richter describes a method of removing heat from the interior of a high-performance lead acid battery by attaching a U-shaped tube to the negative electrode grid and circulating a coolant through the tube.
U.S. Pat. No. 5,035,964 to Levinson, et al., describe attaching a finned heat sink to a battery and positioned the combination in a chimney structure. The finned heat sink produces a convective flow of air in the chimney to cool the battery and extend its life.
All of the above cited references suggest methods of removing heat that requires the addition of auxiliary apparatus to the battery pack. None suggest how this can be accomplished without modifications that, as U.S. Pat. No. 4,115,630 specifically states, result in an unacceptable addition to the total weight of the cell.
In all sealed cells, the discharge capacity of a nickel based positive electrode is limited by the amount of electrolyte, the amount of active material, and charging efficiency. The charge capacity of a NiMH negative electrode is limited by the amount of active material used, since its charge efficiency is very high, nearly a full state of charge is reached. To maintain the optimum capacity for a metal hydride electrode, precautions must be taken to avoid oxygen recombination or hydrogen evolution before full charge is reached. This becomes a critical problem for batteries in any HEV system that undergo repetitive charge and discharge cycles. The problem of venting is not new and many manufacturers have attempted to solve it. Typically the solution has involved the use of a gas consumption electrode (GCE). Typical GCEs are carbon, copper, silver, or platinum prepared in a porous form to provide a large surface area for gas recombination is the site of catalytic oxygen reduction.
U.S. Pat. No. 5,122,426 describes a GCE that has three distinct layers, a hydrophobic electrically non-conductive first layer, a hydrophilic second layer, and a hydrophobic third layer. This third layer is electrically connected to the negative electrode.
Similarly, U.S. Pat. No. 5,128,219 describes a gas consumption electrode comprising a metallic component, such as Pd, Ni, or Cu, and a film of activated carbon, carbon black, and a binder. Use of the described GCE is particularly discussed in a button cell.
While many GCEs are very efficient, their presence decreases the area available for active electrodes and hence decreases the overall volumetric energy density of the cell. In cells of an HEV system like all sealed NiMH cells, it is desirable to keep pressures within acceptable limitations without the necessity of using a GCE.
The foregoing are just a few examples of the differences in battery requirements for EV applications and HEV applications. There are also many other differences depending upon the particular type of HEV system employed. These will be discussed later in connection with particular HEV systems. Given the fundamental differences in requirements between the EV and those for an HEV application, it could be expected that those batteries currently optimized for use in EV applications will generally not be suitable for HEV without increasing power density. While the demonstrated performance of Ovonic EV batteries has been impressive, these cell and battery designs have been optimized for use in pure EVs and therefore do not meet the specific requirements for HEVs.